In many cases, low reservoir permeability is the critical factor limiting production rates. This places a great deal of importance on well testing because the decision of whether to develop an area often hinges on the reservoir permeability. Accurate permeability determination is also important for making intelligent decision about stimulation designs and well spacing.
Slug tests and injection/falloff tests are the two most common methods of testing water saturated coal seams in the Warrior Basin. Slug tests are simpler and more economical to perform than injection/falloff tests, but the practical radius of investigation of a slug test is relatively short. Reservoir heterogeneities are also much more difficult to interpret with a slug test than with a properly performed injection/falloff test. The major disadvantages of injection/falloff tests are the high cost of the tests and if not performed correctly, may provide un-analyzable or misleading data. Injecting at too high of a rate and fracturing the coal has invalidated many injection/falloff tests performed in the Warrior Basin. Even after Zuber, et al. published a paper that indicated the need for low injection rates, fracturing during injection is still common. Problems most often arise when testing thin seams with low permeability. The low permeability seams often require pumping rates well below the commonly used rate of 0.5 gal./min. and the test duration needs to be longer to achieve the same radius of investigation as a test performed in a more permeable reservoir.